1. Field of the Invention
The present invention relates to an imaging apparatus which includes an image sensor. More particularly, the present invention relates to an imaging apparatus that acquires radiation image data by using an image sensor according to a radiation ray transmitted through an object.
2. Description of the Related Art
In recent years, an X-ray imaging system that captures an X-ray image of an object as a digital image has been developed. Compared to conventional X-ray photograph systems that use a silver halide photograph, the X-ray imaging system is more practical in that it can record an X-ray image of an extremely wide exposure range.
More specifically, according to the X-ray imaging system, an X-ray of an extremely wide dynamic range is photoelectrically converted into an electric signal by, for example, phosphor. The obtained electric signal is read out and further converted into a digital signal.
According to the X-ray imaging system, since a digital signal is processed so that an X-ray image is output as a visible image on a recording material such as a photosensitive material or a display device such as a cathode ray tube (CRT), even if an X-ray exposure quantity varies to some extent, a good X-ray image can be obtained.
In an X-ray imaging apparatus used in the X-ray imaging systems, an image sensor such as a charge-coupled device (CCD) image sensor, a metal oxide semiconductor (MOS) image sensor, or a complementary MOS (CMOS) image sensor is used. Especially, the CMOS image sensor has high sensitivity and enables X-ray imaging in a low dose range. For example, Japanese Patent Application Laid-Open No. 2005-143802 discusses a technique concerning a radiation imaging system using a CMOS image sensor.
An imaging area of a large size such as 11 inch×11 inch is necessary for an image sensor used in the X-ray imaging apparatus. However, a size of the CMOS image sensor is generally smaller than the above-described size due to manufacturing process constraints. Accordingly, an appropriate sized imaging area of the CMOS image sensor is configured by putting a plurality of CMOS area sensors together, for example, in a tiled arrangement (in a matrix) to create one large imaging area.
Generally, a dedicated tool is used in putting the CMOS area sensors together. However, when one of the arranged CMOS area sensors needs to be removed for some reason such as a defect, it is difficult to set a new CMOS area sensor in a correct place without misregistration. For purposes of the present specification, the term “misregistration” may be broadly interpreted as a deviation from an intended position of an object, such that a spatial aberration is created. For example, in the abovementioned scenario, if a new CMOS area sensor is not placed in the exact correct location, a spatial aberration is created because the new CMOS area sensor may not be properly aligned with the other CMOS area sensors. Accordingly, when the CMOS area sensors are not aligned, an X-ray image obtained by the X-ray imaging apparatus using such CMOS area sensors may have pixel misregistration. Correction of the pixel misregistration by image processing using software is not practical since a frame rate will be reduced. Thus, it is difficult to repair the area sensors which are put together.